This invention relates to a positioning apparatus, an exposure apparatus and a method of manufacturing a semiconductor device. More particularly, the invention relates to a positioning apparatus for measuring the position of a stage very accurately using light, and adjusting the position of the stage highly precisely based upon the results of measurement, an exposure apparatus having this positioning apparatus, and a method of manufacturing a semiconductor device using these apparatus.
The higher degree of integration applied to semiconductor integrated circuits has been accompanied by greater demand for the formation of ultrafine patterns, and this has led to demand for greater positioning accuracy of reticles and wafers. For this reason, it is required that a scanning-type exposure apparatus have a high positioning accuracy of 0.01 .mu.m or less for a reticle stage which supports a reticle or for a wafer stage on which a wafer is placed for scanning the wafer relative to a projection optical system. A laser interferometer is used to achieve this highly precise measurement.
In measurement of length performed by an interferometer, a fluctuation in the refractive index of air on the optical path of length measurement is a cause of error. The refractive index of air varies depending upon temperature, and the rate of this change is about 1 ppm/.degree.C. Accordingly, in order to make the measurement error less than 0.01 .mu.m when the measured distance is 300 mm, it is required that the variation in the temperature of air be held below 0.03.degree. C.
To achieve this, an XY positioning apparatus for positioning a wafer in the X and Y directions in a step-and-repeat reduced-projection exposure apparatus (stepper) is equipped with an air conditioner. A variation in the temperature of the air on the optical path of length measurement is reduced by air conditioner down-flow from above (the Z direction) or air conditioner side-flow from the side (the X or Y direction).
The down-flow scheme is ideal. However, in a semiconductor exposure apparatus or reticle coordinate measurement apparatus, a projection lens, microscope, focus sensor and other devices are disposed in close proximity to the top table of the XY positioning apparatus. As a result, the down-flow scheme is difficult to realize owing to design limitations. Consequently, the side-flow method is widely employed.
Such an XY positioning apparatus, however, includes a source of heat such as a motor for drive in the X and Y directions. As a consequence, the temperature of the positioning apparatus becomes higher than the temperature of the air flow from the air conditioner. It is known that this causes the air flow to develop an uneven temperature distribution (see the specification of Japanese Patent Application Laid-Open No. 3-129720, and Ishida: O plus E, 1995, October, pp. 84.about.89). If the temperature of the air flow becomes non-uniform, an error in measurement of length occurs. The same problem arises in a scanning-type exposure apparatus.